In recent years, soft silicone hydrogel contact lenses become more and more popular because of their high oxygen permeability and comfort. “Soft” contact lenses can conform closely to the shape of the eye, so oxygen cannot easily circumvent the lens. Soft contact lenses must allow oxygen from the surrounding air (i.e., oxygen) to reach the cornea because the cornea does not receive oxygen from the blood supply like other tissue. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation cause the undesirable growth of blood vessels in the cornea. By having high oxygen permeability, a silicone hydrogel contact lens allows sufficient oxygen permeate through the lens to the cornea and to have minimal adverse effects on corneal health.
Typically, silicone hydrogel contact lenses are produced according to a cast molding technique involving use of disposable or reusable molds and a silicone hydrogel lens formulation (i.e., a mixture of vinylic monomers and/or vinylic macromers). A silicone hydrogel lens formulation often comprises a siloxane-containing vinylic monomer having a tris(trialkylsilyloxy)silylalkyl group (e.g., tris(trimethylsilyloxy)-silylpropyl acrylate, tris(trimethylsilyloxy)-silylpropyl methacrylate, tris(trimethylsilyloxy)-silylpropyl acryalmide, tris(trimethylsilyloxy)-silylpropyl methacrylamide, tris-(trimethylsiloxysilyl) propylvinyl carbamate, etc.), one or more hydrophilic vinylic monomers (e.g., N,N-dimethylacrylamide, N-vinyl pyrrolidone, Hydroxyethyl methacrylate, N-vinylacetamide, N-methyl-3-methylene-2-pyrrolidone, or mixtures thereof), and one or more polysiloxane vinylic monomers/macromers. It is believed that such a tris(trialkylsilyloxy)silylalkyl-containing vinylic monomer can provide resultant silicone hydrogel contact lenses with good optical properties and a high oxygen permeability. However, silicone hydrogel lenses produced from a lens formulation comprising a tris(trialkylsilyloxy)silylalkyl-containing vinylic monomer may not have a desired thermal stability when being stored in an aqueous solution, because monomeric units derived from the tris(trialkylsilyloxy)silylalkyl-containing vinylic monomer are susceptible to hydrolysis.
In addition, tris(trialkylsilyloxy)silylalkyl-containing vinylic monomers are hydrophobic and have limited compatibility with some hydrophilic components in a silicone hydrogel lens formulation. It may not be suitable for use in an environmentally-friendly lens-manufacturing process with minimized use of an organic solvent, i.e., using a water-based silicone hydrogel lens formulation and/or using water in lens extraction process.
Therefore, there is still a need for hydrolytically-stable, hydrophilized silicone-containing vinylic monomers suitable for making silicone hydrogel contact lenses with long thermal stability.